1. Field of the Invention
The present invention relates to improvements to methods for injecting polyurethane foam under high pressure into a hollow envelope through an orifice provided in said envelope.
Polyurethane foam casting heads are known for putting these methods into practice. The operation of these heads generally includes two phases. The first phase is a mixing phase, during which at least two constituents of the polyurethane foam, a polyol and an isocyanate, are brought under high pressure into a mixing chamber, then this mixture flows through a bore towards an outlet, possibly coinciding with the orifice of an envelope to be filled. The second phase is a recycling and cleaning phase during which at least one piston acts for looping on itself each of the two circuits for feeding the components and for scraping the walls of the bore of the mixing chamber, so as to clean it.
2. Description of the Prior Art
In FR No. 81 06162 a method is also described for injecting polyurethane foam of the above defined type, which uses an injection head further having means for bringing under said piston, before the discharge step, a member for scraping the walls of the bore when the piston slides towards the output of the bore, this scraping member being formed by a plate whose dimensions are substantially equal to the dimensions of the section of the bore and of the orifice and comprising one or more flexible lips extending continuously over its perimeter.
In this method the polyurethane foam is cast in the liquid phase in the hollow envelope inside which the expansion or foaming takes place in situ, the envelope being shaped by an external mold.
To provide the best possible sealing, the wall of the envelope must comprise not only an ordinary fabric but in addition a barrier which may be a distinct foam and manufacture of the envelope is expensive and fairly delicate.
The present invention aims at simplifying such manufacture by avoiding the necessity of providing the hollow envelope with special sealing means, or al least by simplifying the characteristics thereof.